Telemetry duty cycle management system for an implantable medical device

ABSTRACT

An implantable medical device comprising a far field RF transmitter and receiver, a controller circuit communicatively coupled to the RF transmitter and receiver, and a wakeup timer circuit integral to, or communicatively coupled to, the controller. The controller is configured to initiate power up of the RF transmitter and receiver during a wakeup interval defined by the wakeup timer circuit, detect a digital key received from a second device during the wakeup interval, transmit a response using the RF transmitter when the digital key is received, and receive a communication from the second device and resynchronize the wake-up timer according to a time of the communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/325,584, filed on Jan. 4, 2006, now issued as U.S. Pat. No.7,738,964, which is a continuation of U.S. patent application Ser. No.10/025,223, filed on Dec. 19, 2001, now issued as U.S. Pat. No.6,993,393, the specifications of which are incorporated herein byreference.

TECHNICAL FIELD

In general, the present subject matter relates to implantable medicaldevices, such as cardiac pacemakers and cardioverter/defibrillators, andin particular, the present subject matter relates to wireless telemetrywith implantable medical devices.

BACKGROUND

Implantable medical devices, including cardiac rhythm management devicessuch as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer, or programmer, via a radio frequency telemetrylink. A clinician may use such a programmer to program the operatingparameters of an implanted medical device. For example, the pacing modeand other operating characteristics of a pacemaker may be modified afterimplantation in this manner. Modern implantable medical devices alsoinclude the capability for bidirectional communication so thatinformation can be transmitted to the programmer from the implanteddevice. Among the data which may be telemetered from an implantablemedical device are various operating parameters and physiological data,the latter either collected in real-time or stored from previousmonitoring operations.

Telemetry systems for implantable medical devices may utilize radiofrequency (RF) energy to enable bidirectional communication between theimplantable medical device and an external programmer. In someapplications, a wireless RF carrier is modulated with digitalinformation, typically by amplitude shift keying where the presence orabsence of pulses in the signal constitute binary symbols or bits. Anexemplary telemetry system for an external programmer and a cardiacpacemaker is described in U.S. Pat. No. 4,562,841, issued to Brockway etal. and assigned to Cardiac Pacemakers, Inc., the disclosure of which isincorporated herein by reference. The external programmer transmits andreceives the radio signal with an antenna incorporated into a wand thatcan be positioned in proximity to the implanted device. The implantablemedical device also generates and receives radio signals by means of anantenna, which may include a wire coil inside of the device casing.

Some RF telemetry systems used for implantable medical devices, such ascardiac pacemakers, utilize inductive coupling between the antennas ofthe implantable medical device and an external programmer in order totransmit and receive wireless signals. Because the induction fieldproduced by a transmitting antenna falls off rapidly with distance, suchsystems require close proximity between the implantable medical deviceand a wand antenna of the external programmer in order to work properly,usually on the order of a few inches. This requirement is inconvenientfor the patient and the clinician, and thereby limits the situations inwhich telemetry can take place.

SUMMARY OF THE INVENTION

The present invention includes a system and method for managing the dutycycles of RF telemetry components in an implantable medical device andan external device in order to lessen power consumption. In accordancewith the invention, the RF telemetry circuitry of both devices ismaintained in a quiescent or powered down state and powered up atpredetermined time intervals. According to one embodiment, in a powereddown state, the electric current is reduced and in a powered up state,the electric current is raised to allow reception or transmission of RFenergy.

One of the devices is designated as the master device and configured totransmit a digital key when the RF circuitry is powered up in an attemptto establish a communications session with the other device. The otherdevice is designated as the slave device and configured to listen forthe digital key when its RF circuitry is powered up and transmit aresponse if the digital key is received. A communications session isthen begun, and the circuitry of both devices remains powered up untilthe session is finished. When no communications sessions are takingplace, the duty cycle of the RF circuitry in each device is thus reducedto only the time spent in transmitting or listening for the digitalsignature. In a preferred embodiment, the external device is configuredas the master device while the implantable medical device is configuredas the slave device.

In addition, the present subject matter includes, among other things, asystem having an implantable medical device having multiple means forwireless communication. In one embodiment, the system communicates usingtwo independent means, each adapted for a particular function. Forexample, one embodiment includes a near field wireless communicationmeans, such as an inductive coupling, as well as a far field wirelesscommunication means, such as a far field RF coupling.

The present subject matter includes, among other things, an apparatusand method for enabling communications with an implantable medicaldevice utilizing a near field inductive coupling and far fieldelectromagnetic radiation. In one embodiment, communications in aclinical setting may be conducted using either the inductive coupling orthe far field radiation coupling. Communications over a greaterdistance, in general, are conducted using the far field radiationcoupling.

In accordance with one embodiment, a conductor coupled to theimplantable medical device serves as an antenna that transmits andreceives far field RF radiation modulated with telemetry data. Theantenna is adapted to transmit far field radiation at a particularfrequency. In one embodiment, a tuning circuit is used to adjust theimpedance, and thereby tune the antenna to a particular frequency. Inone embodiment, a therapy lead of a cardiac rhythm management device,which is otherwise used for stimulating or sensing electrical activityin the heart, has, incorporated therein, a wire antenna. In oneembodiment, a specialized structure is incorporated into such a therapylead in order to isolate a separate antenna section therein. In oneembodiment, the antenna includes a helix structure located proximate to,or internal to, a housing of the medical device. In one embodiment, theantenna includes a wire disposed around the circumference of the outersurface of the device housing.

In one embodiment, a near field coupling includes an implanted coil ofwire. The implanted coil is tailored and aligned for inductive couplingwith an external coil. In one embodiment, the coils are coupled byelectromagnetic coupling.

In one embodiment, the system includes a timer function that allows farfield communications to be enabled for a limited duration. In oneembodiment, for example, a clinician marks the beginning of aprogramming period by transmitting a near field signal using aprogrammer wand. During the programming period, the far field section ofthe implantable medical device is engaged, and powered, and can receiveprogramming instructions without using a wand. At the expiration of theprogramming period, the far field section is disabled. In oneembodiment, the near field section remains continuously available andoperates independent of the far field section. In one embodiment, thedata received by the far field section is installed and executedimmediately after receipt. In one embodiment, after transmitting updatedparameters or program instructions, the clinician then uses the nearfield section to transmit an update command which causes the implantablemedical device to implement the replacement parameters or programinstructions.

Upon receiving an update command at the implantable medical device, thereceived data is transferred from a first memory location to a secondmemory location. Instructions, or data, stored in the second memorylocation controls the operation of the implantable medical device. Forexample, configuration statements or data can be communicated to theimplantable medical device and initially stored in a temporary memorylocation and after receiving an update command, the contents of thetemporary location are transferred to a semi-permanent memory locationfor execution and implementation.

In one embodiment, the near field communication section is used tosignal to the implantable medical device that selected functions areavailable. In various embodiments, the selected functions includeinterrogation functions or programming functions. For example, in oneembodiment, the implantable medical device transmits status informationaccording to a predetermined schedule. In various embodiments, thestatus information is transmitted using either the far fieldtransmitter, the near field transmitter, or both. In one embodiment, thefar field receiver section normally remains in an unpowered mode andafter receiving a near field transmission, the far field receiver ispowered and data received is stored in a memory. In one embodiment, thefar field receiver is ordinarily unpowered and transitions to a poweredmode upon receipt of a near field signal. The receipt of the near fieldsignal marks the beginning of a period during which a predeterminedprogramming or interrogation function is available. During the periodbefore receiving the near field signal, and during the period after thepredetermined period, the programming or interrogation function is notavailable.

Other means of programming, interrogating and communicating with theimplantable medical device are also contemplated. For example, in oneembodiment, a far field communication including a particular messagemarks the opening of a window during which both far or near fieldcommunications may be subsequently conducted.

As another example, one embodiment includes an implanted medical devicehaving a far field transceiver operated according to a predeterminedduty cycle and a programmer having a complementary far fieldtransceiver, also operated according to a predetermined duty cycle. Insuch an embodiment, the medical device and the programmer are configuredto communicate on a peer-to-peer basis and not on the basis of amaster-slave relationship. In one embodiment of the peer-to-peer system,the medical device and the programmer are cycled in phase according to apredetermined schedule. In one embodiment, the medical device isoperated according to a first duty cycle and the programmer is operatedaccording to a second duty cycle or operated continuously.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a telemetry system for an implantablemedical device and an external device.

FIG. 2 is a block diagram view of an embodiment of an implantablemedical device having both a near field module and a far field module.

FIG. 3 is a block diagram of an embodiment of an implantable medicaldevice having a near field transceiver and a far field transmitter.

FIG. 4 is a block diagram of an embodiment of an implantable medicaldevice having a near field transceiver and a far field receiver.

FIG. 5 is a block diagram of an embodiment of an implantable medicaldevice having a near field transceiver and a far field transceiver.

FIG. 6 is a flow chart illustrating a method of transmitting executed byan embodiment of the present subject matter.

FIG. 7 is a flow chart illustrating a method of communicating executedby an embodiment of the present subject matter.

FIG. 8 is a block diagram of a programmer in accordance with the presentsubject matter.

FIG. 9 is a block diagram of an embodiment of an implantable medicaldevice having a far field transceiver.

DETAILED DESCRIPTION

The present invention includes a system and method for providingfar-field RF telemetry between an implantable medical device and anexternal device in which power consumption is lessened by managing theduty cycles of the RF transmitting and receiving components. As usedherein, the term data includes digital data and analog signals.

FIG. 1 shows selected telemetry components of external device 200 andimplantable medical device 100. In this functional block diagram, thecomponents are shown as being substantially identical in each device,however, in one embodiment, the components differ. In this exemplaryembodiment, implantable medical device 100 and external device 200 aremicroprocessor-based devices with implantable medical device 100 havingcontroller 102A and external device 200 having controller 102B.Controllers 102A and 102B each include a microprocessor and memory fordata and program storage that supervises overall device operation aswell as telemetry. In one embodiment, the code executed by controllers102A and 102B also implements the duty cycle management schemes to bedescribed below. Implantable medical device 100, in one embodiment,includes a cardiac rhythm management device such as a pacemaker orimplantable cardioverter/defibrillator, and external device 200 includesa data-gathering device such as an external programmer or remotemonitor. In one embodiment, both device 100 and device 200 are batterypowered. In one embodiment, device 200 is powered by a metered lineservice.

In one embodiment, long-range RF receivers 120A, and 120B, andlong-range RF transmitters 110A, and 110B, are interfaced tomicroprocessors 102A, and 102B, in implantable medical device 100 andexternal device 200, respectively. In one embodiment, transmitters 110A,and 110B, and receivers 120A, and 120B, are coupled to antennas 101A,and 101B, through transmit/receive switches 130A and 130B, respectively.Switches 130A and 130B are controlled by controllers 102A and 102B,respectively. Switch 130A passes RF signals from transmitter 110A toantenna 101A or from antenna 101A to receiver 120A. Switch 130B passesRF signals from transmitter 110B to antenna 101B or from antenna 101B toreceiver 120B. To effect communications between devices 100 and 200, anRF carrier signal modulated with digital data is transmitted wirelesslyfrom one antenna to the other. In one embodiment, a demodulator forextracting digital data from the carrier signal is incorporated intoreceivers 120A and 120B, and a modulator for modulating the carriersignal with digital data is incorporated into transmitters 110A and110B. In one embodiment, an interface coupled to controller 102A and RFtransmitter 110A, and controller 102A and receiver 120A in device 100enables data transfer and also provides a means by which controller 102Acan power up or power down receiver 120A or transmitter 110A and thusmanage duty cycles in the manner described below. In one embodiment, aninterface coupled to controller 102B and RF transmitter 110B, andcontroller 102B and receiver 120B in device 200 enables data transferand also provides a means by which controller 102B can power up or powerdown receiver 120B or transmitter 110B and thus manage duty cycles inthe manner described below. In one embodiment, wakeup timers 180A and180B are coupled to controllers 102A, and 102B, respectively. Timers180A and 180B define the duty cycles and, in one embodiment, areimplemented in code executed by the respective controller and, in oneembodiment, include discrete components.

Far field telemetry over greater distances can be achieved by wirelessRF communication. The ability to communicate over a greater distance maybe advantageous for the patient as well as the clinician. The increasedcommunication range makes possible other applications of the telemetrysystem such as remote monitoring of patients and communication withother types of external devices. To emit a substantial portion of theenergy delivered to an antenna as far field radiation, preferably, thewavelength of the driving signal is not much larger than the length ofthe antenna. Far-field radio frequency communications with an antenna ofa size suitable for use in an implantable medical device therefore canbe conducted using a carrier frequency in the range between a fewhundred MHz to a few GHz. Active transmitters and receivers for thisfrequency range employ special RF components (which may include SiGe orGaAs semiconductor devices) that consume a significant amount of power(typically tens of milliwatts). Implantable medical devices, however,are powered by a battery contained within the housing of the device thatcan only supply a limited amount of continuous power before it fails.When the battery fails in an implantable medical device, it must bereplaced which necessitates a reimplantation procedure. Portableexternal devices may also be battery powered, and recharging orreplacement of the battery in such devices is an inconvenience.

Long-range RF telemetry circuitry (i.e., the transmitter and receiver)typically requires power on the order of tens of milliwatts in order tooperate. Cardiac rhythm management devices in use today, on the otherhand, are usually designed to operate with average power in themicrowatt range. Thus, to meet the power budget of such devices, the RFtelemetry circuitry is preferably duty cycled down by about four ordersof magnitude. An example of duty cycling for an implantable medicaldevice is described in U.S. Pat. No. 5,342,408, presently assigned tothe Guidant Corp. and hereby incorporated by reference. Externaldevices, although not subject to the same power constraints asimplantable medical devices, may be portable and battery powered. Dutycycling down the RF circuitry in external devices may advantageouslyavoid the inconvenience of premature battery depletion. In oneembodiment, implantable medical device 100 employs a power managementscheme in which RF receiver 120A and RF transmitter 110A are duty cycledbased upon synchronized timer wakeups. In one embodiment, externaldevice 200 employs a power management scheme in which RF receiver 120Band RF transmitter 110B are duty cycled based upon synchronized timerwakeups.

In such a scheme, the RF telemetry circuitry is normally in a low powerstate until powered up to transmit or receive a message. In oneembodiment, the controller in the implantable medical device and thecontroller in the external device are both programmed to maintain the RFcircuitry in a quiescent or powered down state and then power up thecircuitry at programmable time intervals based upon timer expirations orother conditions.

In one embodiment, one of the devices is designated as the slave device.After a programmable wakeup time interval, the RF receiver in the slavedevice is powered up for a specified slave time window and listens for avalid digital key transmitted by another device designated as the masterdevice. The master device, after a programmable wakeup time interval,also powers up its RF transmitter and receiver for a specified mastertime window to transmit the digital key and listen for a response fromthe slave device. If no valid digital key is received, the slave devicepowers down the RF receiver until the next wakeup interval. If a validdigital key is received, the slave device powers up its RF transmitterand responds by transmitting a response code to the master device toinitiate a communications session.

In one embodiment, the slave device receives a digital key withoutinitiating a communications session, in which case the reception of thekey is used for timing or other purposes by the slave device. In oneembodiment, different digital keys are used to differentiate betweensituations where the slave is obligated to respond and initiate acommunications session with the master device and situations where theslave has the option to respond or not. If a communications session isestablished, the RF circuitry in both the slave and master devicesremains powered up until the session is finished, and then the RFcircuitry in each device is powered down until the next wakeup interval.

In one embodiment, the external device is configured as the masterdevice while the implantable medical device is configured as the slavedevice. This configuration may be less burdensome on the implantablemedical device's more limited battery power supply since periodicallypowering up the RF receiver to listen for a digital key may draw lesscurrent than powering up both the RF transmitter and RF receiver totransmit a digital key and listen for a response. In addition, incertain situations, it may be undesirable for the implantable medicaldevice to transmit an RF signal. For example, a patient may be onboard acommercial airplane or in a country where regulations prohibit RFtransmissions at the system frequency. If the implantable medical deviceis the slave, then unless a digital key is received, the implantablemedical device will not broadcast an RF transmissions.

In one embodiment, the programmable wakeup time intervals for the masterand slave devices are synchronized so that they occur simultaneously. Aslong as the devices are located within range, a communications sessionwould then be established at every such wakeup interval.

In one embodiment, synchronization of the wakeup timers is performedduring any communications session established with the RF telemetrylink. Synchronization can be done by a variety of means. In oneembodiment, synchronization entails having one device transmit a timestamp and having the other device adjust its timer accordingly. In oneembodiment, the receiving device synchronizes by detecting a start of amessage and realigning its wakeup interval to begin at the same time. Inone embodiment, synchronization of the timers is performed usinginductive coupling. In one embodiment, an inductively coupled link isused to transmit a magnetic pulse that activates a switch or isotherwise detected by the receiving device, and the device is thenprogrammed to synchronize its wakeup timer at that time by setting it toa predetermined value.

Another alternative is for a communications session to be establishedwith an inductively coupled link such as used in conventionalshort-range telemetry. FIG. 1 shows inductively coupled transceivers140A and 140B, each coupled to antennas 150A and 150B, respectively, foreach of the implantable medical device and external device. Transceivers140A and 140B, and antennas 150A and 150B, draw low power, and in oneembodiment, are operated continuously. In this manner, a communicationslink can be established immediately without waiting for a wakeupinterval.

Whatever means are used to establish a communications session, thewakeup timers of the master and slave devices can be synchronized and aprogrammable wakeup interval agreed upon by the two devices during thesession. The wakeup interval can be a fixed value set by clinician inputor can be made to vary in accordance with detection of particularevents. For example, when the implantable medical device includes acardiac rhythm management device and an arrhythmia is detected, theimplantable medical device can be programmed to increase the frequencyat which communications sessions occur in order to more closely monitorthe patient. This is accomplished by the implantable medical devicereducing the wakeup interval for each device when a communicationssession with an external device is next established.

In one embodiment, the frequency of communications sessions is increasedby reducing the wakeup interval when the implantable medical devicedetects a low battery condition or a change in lead impedance.Increasing the frequency of communication sessions allows the operatingstatus of the implantable medical device to be more frequentlycommunicated to the external device. In one embodiment, clinician inputfrom an external programmer can be used to change the wakeup interval ofthe implantable medical device. In one embodiment, the wakeup intervalis varied pseudo-randomly from one communications session to another.Pseudo-random variability may be desirable in order to minimize thechance of periodic interference from an external source or other devicescausing transmission problems.

In one embodiment, each slave device has a unique digital key and asingle master device can thus, individually, access each slave. Thisconfiguration prevents two slave devices from interfering with eachother by responding to a digital key transmitted from the master device.In one embodiment, a slave device is programmed to respond to a digitalkey used in common with one or more other devices. For example, in oneembodiment, a digital key is uniquely specified for a family of slavedevices. In that case, the possibility of interference can be lessenedby programming the slave device to respond to a digital key after arandom delay time.

In one embodiment, the timing of the master device is perfectlysynchronized with the timing of the slave device. That is, the power uptimes for the receiver in the slave device and both the transmitter andreceiver of the master device are precisely defined. Precise definitionof the power up times allows the duty cycles of the RF circuitry to beminimized, thus reducing power consumption.

It may be found that timing drift prevents perfect synchronization ofthe wakeup intervals. In one embodiment, the master time window beginsat some time before the slave time window starts. That is, the masterdevice begins transmitting the digital key shortly before the slavedevice is expected to be listening for it. In one embodiment, the mastertime window extends beyond the slave time window such that if noresponse from the slave is received, the master continues transmittingthe digital key until after the slave is expected to no longer belistening. In one embodiment, the listening time of the slave or slavetime window is great enough to receive at least two digital keys so thatif it just misses the start of a key transmitted by the master, the nexttransmitted key will be received in its entirety. This technique thusincreases the probability that communication between the devices will beestablished even when timing drift occurs. Because the master devicetransmits and receives for a longer period of time than the slave devicelistens, there is a greater energy burden on the master device. In oneembodiment, the implantable medical device, with its more severe batteryconstraints, is configured as the slave device.

In one embodiment, the wakeup timers are synchronized (orresynchronized) automatically during each communication session.Resynchronizing during each communication session reduces the amount ofdesynchronization that exists between the timers of the master and slavedevices (due to timing drift). In one embodiment, the device receivingthe time stamp attempts to compensate for the amount of drift thatoccurred since the last communications session. At each such session,the device receiving the time stamp stores the amount by which it had toadjust its wakeup timer and thus learns the average amount of timerdrift that occurs between the timers of both devices. The device isprogrammed to set its wakeup timer either ahead or behind that of thewakeup timer of the other device if it was slow or fast, respectively,relative to that timer. The amount by which the timer is set ahead orbehind that of the other timer can be made either equal to the amount ofdrift or some fraction of it.

In one embodiment, the master device adjusts the time window used totransmit the digital key and listen for a response at each wakeupinterval based on the average amount of timer drift. In one embodiment,the master device adjusts the time window based on the time since thelast resynchronization. In one embodiment, the master device beginstransmitting the digital key before it anticipates that the slave devicewill be listening for it, and continues transmitting the key until afterit anticipates that the slave device has powered down its receiver. Thishelps to ensure that the master device will catch the slave devicelistening even if the timers of the two devices have drifted relative toone another. The amount of drift between the two wakeup timers willincrease as a function of the time since the last resynchronization. Theamount of energy that the master device spends in attempting toestablish a communications session can therefore be reduced by makingthe duration of the transmitting and listening time window a function ofthe time since the last resynchronization.

Consider the following example. Assume that the wakeup timers of themaster and slave devices were last resynchronized 100 seconds ago. Atthe next wakeup interval, the master device could anticipate that thedrift of the wakeup timers would be rather small, and therefore, themaster device can reliably begin sending the digital key to the slavedevice only a few tens of microseconds before it anticipates that theslave will be listening. On the other hand, if the wakeup timers of themaster and slave devices were last resynchronized eight hours (28,800seconds) ago, then it is likely that the drift will be 288 times greaterthan had it been resynchronized 100 seconds ago. Thus, at the nextwakeup interval, the master device should begin sending the digital keyto the slave device a few tens of milliseconds before it anticipatesthat the slave will be listening.

In one embodiment, the master device is configured to conduct a searchfor a non-synchronized slave receiver. A slave receiver may benon-synchronized for any of a number of reasons. For example, but not byway of limitation, the slave receiver may be newly implanted in aparticular patient, the batteries of the master device may have justbeen changed, or the master device may not have been used recently. Inone embodiment, the master and slave devices are resynchronized using aninductive telemetry link as described above.

Consider an example where the master device has not successfullyestablished a communications session with a particular slave device fora specified maximum period of time (e.g., 4 hours). In this case, themaster device will assume that the slave device is not synchronized, andthe master device will begin a non-synchronized receiver search. In oneembodiment, the master device continuously transmit its digital key andlisten for a response until either a communications session isestablished or a programmable maximum non-synchronized search interval(e.g., 200 seconds) passes. If a communications session is established,the wakeup timers of the two devices are resynchronized. If the maximumnon-synchronized search interval passes with no communications sessionbeing established, then the master device assumes that no slave deviceis within range and goes back to its previous wakeup interval pattern.If the specified maximum time with no communications session beingestablished again passes (e.g., another 4 hours), the non-synchronizedreceiver search is repeated. In one embodiment, the master device keepstrack of the expected slave device wakeup windows during anon-synchronized receiver search so that the master device can return tothese if no non-synchronized slave device is found.

In the systems described above, the RF circuitry of both the implantableand external devices are duty cycled in order to lessen powerconsumption, with one device operating as a slave and the other deviceoperating as a master. In one embodiment, the devices are programmedsuch that one device is always configured as the master device while theother device is programmed to always be configured as the slave device.In one embodiment, the devices are programmed to dynamically configurethemselves as either master or slave devices depending uponcircumstances. For example, in one embodiment, both the implantable andexternal devices are programmed to normally act as slave devices suchthat they both wake up at periodic intervals to listen for acommunications session request from the other device. Upon a change incircumstances, such as clinician input to the external device ordetection of a particular event by the implantable medical device, oneof the devices configures itself as a master to request a communicationssession at the next wakeup interval. In order to deal with the situationwhere both devices configure themselves as master devices, in oneembodiment, the system incorporates a mediation scheme like thoseemployed in peer-to-peer networks.

In one embodiment, the RF circuitry of the master device is duty cycledand that of the slave device is continuously powered. In one embodiment,the RF circuitry of the slave device is duty cycled and that of themaster device is continuously powered. Such a configuration may bebeneficial, for example, if one device has unusually severe batteryconstraints or is intended for very limited use while the other devicehas access to continuous power. In such a system, the duty cycled deviceis configured to operate as either a master or slave device inestablishing communications sessions with the continuously powered updevice. In one embodiment, the duty cycled device is configured as aslave in order to lessen its power requirements somewhat. Thecontinuously powered device, acting as a master, then periodicallypowers up its RF circuitry at programmed wakeup intervals in order toattempt to initiate a communications session with the duty cycleddevice.

FIG. 2 illustrates an embodiment of an implantable medical device 205having telemetry module 210 coupled to medical module 240. In oneembodiment, implantable medical device 205 is housed in a sealedcanister suited for implantation in a human body. Telemetry module 210includes near field module 220 and far field module 230. Near fieldmodule 220 and far field module 230 are coupled together by telemetrydata bus 215. In various embodiments, bus 215 includes a digital databus or an analog signal line. Near field module 220 is coupled toantenna 225, herein depicted by a loop, or coil. In one embodiment,antenna 225 includes an inductive loop. Far field module 230 is coupledto antenna 235, herein depicted by an RF antenna. In one embodiment,antenna 235 includes a dipole antenna. In one embodiment, antenna 235includes a monopole antenna.

Antenna 235 may include a circumferential antenna disposed around anexterior surface of the device housing. Circumferential antennastructures are described in co-assigned U.S. patent application Ser. No.09/921,653, filed Aug. 3, 2001, CIRCUMFERENTIAL ANTENNA FOR ANIMPLANTABLE MEDICAL DEVICE, now issued as U.S. Pat. No. 6,456,256, andincorporated herein by reference in its entirety.

Near field module 220, in one embodiment, includes an inductivelycoupled transmitter/receiver 140A, as described relative to FIG. 1. Farfield module 230, in one embodiment, includes RF receiver 120A, RFtransmitter 110A and T/R switch 130A, as described relative to FIG. 1.In one embodiment, antenna 225 includes the structure described relativeto antenna 150A of FIG. 1. In one embodiment, antenna 235 includes thestructure described relative to antenna 101A of FIG. 1. In oneembodiment, a switch is provided to select, and therefore communicateusing, either near field module 220 or far field module 230.

According to one embodiment, far field communications conducted by farfield module 230 are preferably conducted using an industrial,scientific, medical (ISM) band. One ISM band is between approximately902 and 928 MHz, however frequencies above or below this figure are alsopossible. At another ISM band of frequencies, approximately 2.45 GHz,the effects of tissue absorption may undesirably attenuate the far fieldsignal, and thus, limit the effective communication range. In someregions of the world, such as for example, Europe, frequencies near 868to 870 MHz SRD (short range device) band may be available and thus, inone embodiment the implantable medical device and external devicecommunicate at a frequency of 869.85 MHz. In one embodiment, the farfield communication frequency is at the Medical Implant CommunicationsService (MICS) band, between approximately 402 and 405 MHz.

Medical module 240 is coupled to telemetry data bus 215 by interface250. In one embodiment, interface 250 includes telemetry direct memoryaccess (DMA) and data buffers. In the embodiment shown, interface 250 iscoupled to system data bus 245. Bus 245 is further coupled tomicroprocessor 260, device random access memory (RAM) 270, devicetherapy circuit 280 and telemetry control circuit 290. In oneembodiment, bus 245 includes a multiple conductor digital data bus. Inone embodiment, microprocessor 260 includes the structure describedrelative to ΦP 102A and wakeup timer 180A of FIG. 1.

In various embodiments, microprocessor 260 includes a digital or analogprocessor adapted for managing data or signals on bus 245. Device RAM270 provides storage for data or programming for execution bymicroprocessor 260. In one embodiment, device therapy circuit 280includes a pulse generator or other therapeutic system. In oneembodiment, therapy circuit 280 includes a monitoring circuit adaptedfor monitoring the condition or performance of an organ or otherphysiological parameter. In one embodiment, device therapy circuit 280includes circuitry for monitoring the condition of implantable medicaldevice 205. For example, in one embodiment, device therapy circuit 280provides a signal based on remaining battery capacity to microprocessor260. Telemetry control circuit 290 includes circuitry adapted forcontrolling telemetry functions relative to the modules coupled totelemetry data bus 215.

FIGS. 3, 4 and 5 illustrate exemplary embodiments of implantable medicaldevice 205, each of which includes a medical module 240 coupled totelemetry data bus 215 as previously described. For example, in FIG. 3,implantable medical device 205A is shown having near field transceiver220A and far field transmitter 230A. In one embodiment, near fieldmodule 220 includes near field transceiver 220A. As a further example,FIG. 4 illustrates implantable medical device 205B having near fieldtransceiver 220A and far field receiver 230B. In one embodiment, farfield module 230 includes far field receiver 230B. As yet a furtherexample, FIG. 5 illustrates implantable medical device 205C having nearfield transceiver 220A and far field transceiver 230C. In oneembodiment, far field module 230 includes far field transceiver 230C.Other embodiments are also contemplated, such as, for example, a nearfield transmitter or receiver coupled to any of transmitter 230A,receiver 230B or transceiver 230C.

Consider next the operation of the present system. At the time ofimplantation, implantable medical device 205 may be programmed andcontrolled by a medical programmer, or external device, adapted tocommunicate using near field transceiver 220A. An embodiment of anexternal device is illustrated in FIG. 1. The programmer may include aflexible wand having a transceiver antenna that communicates with nearfield transceiver 220A. For example, in one embodiment, the programmerincludes an inductive loop antenna and near field transceiver 220A alsoincludes an inductive loop antenna. Following implantation of thedevice, subsequent programming and controlling may also be accomplishedusing near field transceiver 220A.

The embodiment illustrated in FIG. 3 allows implantable medical device205A to transmit data using a far field transmission. For example, inone embodiment, the data includes operational conditions or parametersconcerning implantable medical device 205A or the patient in whichdevice 205A is implanted. In one embodiment, data is transmitted usingfar field transmitter 230A according to a schedule having apredetermined duty cycle or according to a programmed schedule. In oneembodiment, data is transmitted using far field transmitter 230A uponthe occurrence of a predetermined event or when a particular parametricvalue is exceeded. In one embodiment, data is transmitted using farfield transmitter 230A in response to receiving a particular signal vianear field transceiver 220A. In various embodiments, far fieldtransmissions using transmitter 230A proceed until finished, for apredetermined period of time, or until a terminate signal is receivedvia near field transceiver 220A.

The embodiment illustrated in FIG. 4 allows implantable medical device205B to receive data using a far field transmission. For example, in oneembodiment, the data includes programming information or parameters forimplementation by implantable medical device 205B. The parameters mayconcern therapy for the patient in which implantable medical device 205Bis implanted. In one embodiment, data is received using far fieldreceiver 230B according to a schedule having a predetermined duty cycleor according to a programmed schedule. In one embodiment, data isreceived using far field receiver 230B upon the occurrence of apredetermined event or when a particular parametric value is exceeded.In one embodiment, data is received using far field receiver 230B inresponse to receiving a particular signal via near field transceiver220A. In various embodiments, far field transmissions received usingreceiver 230B proceed until finished, for a predetermined period oftime, or until a terminate signal is received via near field transceiver220A or far field receiver 230B.

The embodiment illustrated in FIG. 5 allows implantable medical device205C to both receive data and transmit data using a far fieldtransmission. For example, in various embodiments, far field received ortransmitted data includes programming information or parameters forimplementation by implantable medical device 205C, concerning device205C, or the patient in which device 205C is implanted. The parametersmay concern therapy for the patient in which device 205C is implanted.In various embodiments, data is transmitted or received according to aschedule having a predetermined duty cycle or according to a programmedschedule. In one embodiment, data is received or transmitted using farfield communication upon the occurrence of a predetermined event or whena particular parametric value is exceeded. In one embodiment, data iscommunicated using far field transceiver 230C in response to receiving aparticular signal via near field transceiver 220A. In variousembodiments, far field communications proceed until finished, for apredetermined period of time, or until a terminate signal is receivedvia near field transceiver 220A or far field transceiver 230C. Far fieldtransceiver 230C includes a far field transmitter and a far fieldreceiver and in one embodiment, the far field transmitter communicatesusing a communication protocol that differs from that of the far fieldreceiver.

In various embodiments, the communication modules enable one or moremodes of communicating. The plurality of communication modes may eachhave full redundancy or each may provide different capabilities. Forexample, in one embodiment, a first communication mode supportsreceiving operational parameters and a second communication modetransmits diagnostic data.

FIG. 6 illustrates a method implemented by one embodiment of the presentsystem. In the figure, method 300 concerns the transmission of data froman implantable medical device having a plurality of transmitters ortransceivers. Beginning at 310, the method includes receiving orgenerating data at the implantable medical device, as noted at 320. Forexample, in one embodiment, data is received from electrodes or leadsthat terminate in various portions of the body and are coupled to theimplantable medical device. In one embodiment, the data is processed bythe implantable medical device and the results generated are stored andavailable for telemetering to a remote programmer. At 330, a transceiveris selected for transmitting the data to the remote programmer. In oneembodiment, a transmitter is selected at 330. The selected transmitter,or transceiver, may include a near field transmitter, a far fieldtransmitter, or a combination of near and far field.

Continuing with the figure, at 340, the selected transceiver is poweredon. In one embodiment, the process of turning on the selectedtransceiver includes entering an “awake” mode after departure from a“sleep” mode. At 350, an outbound signal, including the digital datapreviously stored, is wirelessly transmitted by the transmitter. Theprocess ends at 360. The process may also be performed in an order whichdiffers from that illustrated.

FIG. 7 illustrates method 370 which may be useful in programming orconfiguring an implantable medical device. Beginning at 380, a nearfield signal is received as illustrated at 390. In one embodiment, thenear field signal marks the beginning of a time period, or window,during which the far field communication capabilities of the implantablemedical device are available. In one embodiment, during predeterminedtimes after the window, communications are conducted using the far fieldcommunication capabilities of the device. Upon receiving a signal usingthe near field link, the far field window remains open. The duration ofthe time period is monitored by a timer and at 400, the timer isstarted. At 410, following the start of the timer, the far fieldtransceiver is powered on or otherwise enabled. At 420, wirelesscommunications between the implantable medical device and the remoteprogrammer are conducted using the far field transceiver. At 430, dataencoded in the received signal is stored in memory coupled to theimplantable medical device and outbound data is stored in memoryaccessible to the external programmer. In various embodiments, the dataincludes replacement programming or operating parameters that controlsthe operation of the implantable medical device. An inquiry as to thestatus of the timer occurs at 440 after which processing branches to 420if the predetermined time period has not lapsed, or continues to 450 ifthe period has lapsed. At 450, the far field communication capabilitiesare terminated, and in one embodiment, this entails turning off power tothe far field transceiver. The process ends at 460.

In one embodiment, the method illustrated in FIG. 7 includes checkingfor a terminate command in lieu of checking the timer for expiration. Ifa terminate command is received, either by the far field link or thenear field link, then the far field transceiver is powered off.Otherwise, communications using the far field transceiver continuesuntil receipt of a termination command. The termination command may savebattery resources of the device by powering off the far fieldtransceiver in advance of a signal from the timer.

FIG. 8 illustrates a programmer, or external device, in accordance withone embodiment of the present system. Programmer 500 includes near fieldmodule 520 and far field module 530, each coupled to microprocessor 560.Near field module 520 is coupled to antenna 525, here illustrated as aloop antenna suitable for inductive coupling. Far field module 530 iscoupled to antenna 535, here illustrated as an antenna, and in variousembodiments, includes a monopole or a dipole antenna. Microprocessor 560is coupled to input/output module 575 and memory 570. In variousembodiments, input/output module 575 includes a keyboard, bar codescanner, monitor, audio speaker, microphone, printer, storage media,wireless communication modules, or other device. In one embodiment,input/output module 575 provides an interface with which a humanoperator can communicate with an implantable medical device. Memory 570provides storage for data and programming used by microprocessor 560. Inone embodiment, memory 570 provides storage for data to be transmittedto, or received from, an implantable medical device.

In one embodiment, programmer 500 includes both a far field and nearfield communication module, as illustrated, with each module operatingindependent of the other. In one embodiment, programmer 500 includeseither a far field communication module or a near field communicationmodule.

Exemplary System

By way of example, and not by way of limitation, an exemplary system isdescribed as follows.

In this embodiment, an inductive coupling provides a near fieldcommunication link. The inductive coupling is continuously availablesince it draws little or no power when receiving and is able to receivepower from an external programmer for purposes of operating theimplantable medical device. In addition, the inductive coupling isadapted to convey recharging power to the battery of the implantablemedical device. In various embodiments, the near field communicationlink includes an optical link, an audible link, or an infrared link.

In one embodiment, the far field communication link draws power at arate that is greater than that of the near field link. Also, in oneembodiment, transmitting a far field signal draws a greaterinstantaneous current than does receiving a far field signal, however,the far field transmissions are of such short duration that receivingfar field transmissions draws a greater total power. Consequently, farfield reception is available according to a duty cycle. By way ofexample, the ability to receive a far field transmission exists for ashort period of time each minute.

The ability to communicate using a plurality of modes, such as a farfield and near field mode, may provide advantages that are not otherwiseavailable with a single communication mode. For example, theavailability of a second communication mode increases the likelihoodthat the programmer and implantable medical device can communicatereliably. In situations where one mode of communication is unavailable,it may be that another communications mode is available. In addition,whereas the far field communications means may operate according to aduty cycle, the near field communications link is continuouslyavailable, and therefore, available on demand. Furthermore, in someregions of the world, the ability to conduct far field communicationsmay be restricted, and therefore, the availability of a near field linkallows uninterrupted communications.

Using the far field communication link, one embodiment of the presentsystem transmits a signal that may be translated as either “I'm OK” or“I'm not OK.” In one embodiment, after receiving a “not OK” signal, theremote programmer commences a procedure whereby the patient, a doctor,or other responsible party, is contacted and, if warranted, the patientis invited to seek medical attention. Contacting the patient, doctor, orother responsible party may entail placing a telephone call,transmitting a pager message, sending an e-mail, broadcasting a radiomessage, or otherwise attempting to establish communications.

The present system also may find application in the operating room atthe time of implantation of the medical device. For example, at the timeof implantation, one embodiment allows using the far field link forinitial configuration, diagnosis and set up procedures. Enhancedsterility, increased flexibility, and reduced burden are possibleadvantages achievable by reducing the reliance on the traditionalprogrammer wand. Furthermore, one embodiment of the present systemallows remote follow-up with the patient. This may reduce or eliminatethe need for periodic visits to a clinic, thereby reducing system costs.

In one embodiment, security measures are implemented to assure thataccess to the implantable medical device is limited to authorized users.For example, in one embodiment, data transmitted from the implantablemedical device is encrypted, or otherwise encoded, to restrict access.In one embodiment, encryption includes use of a key encryption systemutilizing both a public key and a private key. In one embodiment, theimplantable medical device is programmed to respond only to instructionsfrom an authorized programmer. In one embodiment, the security system isimplemented by means of a programming window of time that is triggeredby an inductively coupled programmer wand.

In one embodiment, the near field link is continuously available and isused to “ping” the implantable medical device. The near field link is beused to establish communications and in a seamless transition, the farfield link continues to communicate with the device after removal of thewand from the vicinity of the implantable medical device. For example,in one embodiment, a wand having an inductive coil is temporarilybrought near the chest wall of the patient in a clinical setting, andupon removal of the wand from near the patient, communications continueusing a far field link. At the conclusion of the session, marked byreceipt of a signal received via the far field link, or upon a signaltransmitted from the near field wand, the far field link is disabled. Inone embodiment, the far field link is used to transmit new parameters orinstruction code to the implantable medical device and the newparameters or code are triggered for implementation upon receipt of acommand transmitted via the inductive coil or far field link.

Alternate Embodiments

In various embodiments, all telemetry with the implanted medical deviceis communicated using a far field transmitter, far field receiver, or afar field transceiver. FIG. 9 illustrates an embodiment of an implantedmedical device having medical module 240 coupled to a far fieldtransceiver 230D via telemetry data bus 215. The telemetry system,including far field transceiver 230D, of the implanted medical device isoperated according to a duty cycle and the duty cycle is compatible withthe operation of a suitable programmer. For example, the programmer mayalso be operated according to a complementary duty cycle or operatedcontinuously.

In one embodiment, the medical device and the programmer communicate ona peer-to-peer basis and not on the basis of a master-slaverelationship. For example, in one embodiment, the medical device and theprogrammer are cycled in phase according to a predetermined schedule. Inthis embodiment, the medical device and the programmer operate withoutestablishing a master-slave relationship wherein one unit is superior tothe other unit. By way of an example, in a system with a fieldcommunication link, the implanted medical device may attempt to receivedata, or transmit data, without having first received an acknowledgmentor ready signal from the programmer. In one embodiment, the implantablemedical device and the programmer engage in a handshaking routinewherein each device has substantially equal autonomy.

The duty cycle of the implantable medical device may correspond to thatof the programmer. For example, a 10% duty cycle of a medical devicebased on a 10 second period (that is, the device may transmit, orreceive, a wireless signal for one second followed by nine seconds ofdormancy) would be able to communicate with a programmer operated on a10% duty cycle based on a 5 second period (that is, the programmer isactive for one second followed by four seconds of dormancy). The medicaldevice may also communicate with a programmer that is continuouslyactive. In one embodiment, the communication link of the medical deviceis operated according to a duty cycle providing one second of activityfollowed by several seconds of dormancy.

In one embodiment, the duration of the window during which far fieldcommunications may be conducted, is dynamically adjustable. For example,after having established a communication link during the active periodof a duty cycle, the programmer may instruct the medical device tomaintain the far field communication link for a programmable period oftime which differs from a duty cycle of the medical device. In this way,for example, the programmer can conduct uninterrupted communicationswith the medical device. In addition, the programmer may instruct themedical device to terminate the far field communication link at apredetermined time and without regard for the duty cycle.

In one embodiment, a near field link is used to mark the beginning of asession during which the far field communication link is available. Theduration of the subsequent far field communication session may bedetermined by a duty cycle, fixed time period or an instruction orparameter received during the communication session. For example, in oneembodiment, the far field communication link is available according to apredetermined duty cycle. Thus, the far field link is terminated uponthe expiration of a predetermined window. As another example, the farfield communication link may be available during the course of a medicalprocedure, such as, device implantation. In such an embodiment, the farfield link may become available at the start of the procedure afterfirst having been triggered by a near field communication link. The farfield link may remain available for a predetermined duration, typicallysufficient to allow completion of a particular medical procedure. Inanother example, the far field communication link is terminated uponreceipt of a predetermined command or instruction received via the farfield link or near field link. A terminate command may truncate the dutycycle that otherwise would determine the window duration.

In some regions of the world, or in certain locations, regulations maypreclude the use of far field radio frequency transmissions. Forexample, in Japan, certain frequencies that are readily available in theUnited States of America are not available for far field RFtransmissions. As another example, Federal Aviation Regulations enforcedby the Federal Aviation Administration (FAA) prohibit the use of certainfar field transmissions while airborne.

In such cases, these restrictions or regulations are satisfied by anembodiment of the present system having a configuration that does nottransmit a far field RF signal unless it first receives a far field RFsignal. In one embodiment, the near field link or far field link firstreceives a wake up signal before the far field transmitter is operated.In one embodiment, the implantable medical device receives and transmitsusing the same communication link. For example, if a far field signal isreceived, then a far field signal is transmitted. The far field signalmay include an acknowledgment.

In one embodiment, the programmer is burdened with the task ofarbitrating the communication links. The programmer is considered morerobust since power may be derived from a metered line service orbatteries that can be replaced without surgical procedures. In variousembodiments, the external programmer is operated continuously oraccording to a duty cycle.

For example, the external programmer may be used with many differentimplantable medical devices, each having a unique identification code.The far field transmitter of the programmer may be operated continuouslyand the transmitted signal can be tailored to correspond with aparticular implantable medical device by proper selection of theidentification code. To establish a communication session with aparticular implantable medical device, the programmer may continuouslytransmit a key and listen for a response. In this manner, the programmerneed not know precisely the timing sequence and duty cycle of eachindividual implantable medical device. After the targeted implantablemedical device wakes up and receives the far field transmission, thedevice transmit a far field acknowledgment signal.

In one embodiment, the duty cycle of the implantable medical device canbe dynamically adjusted. For example, a low power consumption duty cyclemay be operative during times when the implantable medical device isaway from a medical facility, a medium power consumption duty cycle maybe operative during times near a medical facility and a high powerconsumption duty cycle may be operative during a clinician visit at amedical facility. The level of power consumption may be associated withdifferent duty cycles. For example, in one embodiment, the low powerconsumption duty cycle provides that the far field receiver is operatedfor one time period in 10,000 time periods whereas the far fieldreceiver is operated continuously in the high power consumption dutycycle. In one embodiment, the particular duty cycle is selected based onreceiving an external signal (near field or far field) or by an internalparameter detected by the implantable medical device.

In one embodiment, synchronization includes receiving a magnetic signal.The magnetic signal may be received from a wand coupled to a programmer.The signal may correspond to an edge or transition of a magnetic fieldstrength or magnetic alignment. In one embodiment, the implantablemedical device responds to a minimum magnetic field strength.

CONCLUSION

Other embodiments of the present system are also contemplated. Forexample, in one embodiment, the implantable medical device, orprogrammer, has more than two transmitters, transceivers, or receivers.In one embodiment, an implantable medical device is adapted to include asingle transceiver having a plurality of operational modes wherein onemode includes transmitting, or receiving, a substantial signal at a nearfield strength and a second mode includes transmitting, or receiving, asubstantial signal at a far field strength.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. An implantable medical device (IMD) comprising: a far field RFtransmitter and receiver; a controller circuit communicatively coupledto the RF transmitter and receiver; and a wakeup timer circuit integralto, or communicatively coupled to, the controller, wherein thecontroller circuit is configured to: initiate power up of the RFtransmitter and receiver during a wakeup interval defined by the wakeuptimer circuit; detect a digital key received from a second device duringthe wakeup interval; transmit a response using the RF transmitter whenthe digital key is received; and receive a communication from the seconddevice and resynchronize the wake-up timer according to a time of thecommunication.
 2. The IMD of claim 1, wherein the controller circuit isconfigured to: calculate an average of the amount the wakeup timercircuit is adjusted for resynchronization; and adjust the wakeup timercircuit using the calculated average.
 3. The IMD of claim 1, wherein thecontroller circuit is configured to: determine an amount of time sincethe previous communication with the second device; and adjust the wakeuptimer circuit using the determined time.
 4. The IMD of claim 1, whereinthe controller circuit is configured to resynchronize the wakeup timercircuit by aligning the wakeup interval to the start of thecommunication.
 5. The IMD of claim 1, wherein the controller circuit isconfigured to resynchronize the wakeup timer circuit by setting thewakeup timer ahead or behind the start of the communication.
 6. The IMDof claim 1, wherein the wakeup interval timed by the wakeup circuit isdifferent from the wakeup interval of the second device.
 7. The IMD ofclaim 6, wherein the wakeup interval is long enough to receive aplurality of digital keys.
 8. The IMD of claim 1, wherein the controllercircuit is configured to: detect a different digital key received from asecond device during the wakeup interval; and resynchronize the wake-uptimer according to a time the different digital key was received.
 9. TheIMD of claim 1, wherein the controller circuit is configured to changethe wakeup interval according to the communication from the seconddevice.
 10. The IMD of claim 1, wherein the controller circuit isconfigured to shorten the wakeup interval when a low battery conditionis detected.
 11. The IMD of claim 1, wherein the controller circuit isconfigured to initiate transmitting data in the response beforereceiving a message indicating that the second device is ready.
 12. TheIMD of claim 1, wherein the controller circuit is configured to receivedata from the second device without receiving an acknowledge of theresponse.
 13. The IMD of claim 1, wherein, upon detection of a specifiedevent, the controller circuit is configured to communicate as a masterdevice according to a specified master/slave communication protocolduring the wakeup interval.
 14. The IMD of claim 13, wherein thecontroller circuit is configured to: transmit a digital key to thesecond device; establish a communications session with the second devicewhen a response to the digital key is received from the second device;and initiate transmission of a terminate command to end thecommunications session.
 15. The IMD of claim 1, wherein the controllercircuit is configured to enable the far field transmitter and receiverfor an interval of time indicated in the communication from the seconddevice.
 16. A method comprising: supplying power to a far field RFtransmitter and receiver of an IMD according to a wakeup intervaldetermined by a device wakeup timer; detecting a digital key receivedfrom a second device during the wakeup interval; transmitting a responseusing the RF transmitter when the digital key is received; receiving acommunication from the second device; and re-synchronizing the devicewake-up timer according to a time of the communication.
 17. The methodof claim 16, wherein re-synchronizing the device wake-up timer includesaligning the wakeup interval to the start of the communication.
 18. Themethod of claim 16, wherein re-synchronizing the device wake-up timerincludes setting the wakeup timer ahead or behind the start of thecommunication.
 19. The method of claim 16, including communicating withthe IMD as a master device according to a specified master/slavecommunication protocol during the wakeup interval upon an IMD detectionof a specified event.
 20. The method of claim 19, wherein communicatingwith the IMD as a master device includes: initiating communications withthe second device; establishing a communications session with the seconddevice when a response is received from the second device; andtransmitting a terminate command to end the communications session.